The new 3GPP Rel-13 study item “Licensed Assisted Access” or LAA allows Long Term Evolution, LTE, equipment to use unlicensed 5 GHz spectrum as a complement to the licensed spectrum. With LAA, devices connect to a primary cell or PCell in the licensed spectrum and a secondary cell or SCell in the unlicensed spectrum. By aggregation of the licensed and unlicensed carriers, the User Equipment, UE, benefits from the additional transmission capacity provided by the unlicensed spectrum. To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the PCell is simultaneously used in the SCell.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without first performing some type of channel sensing. That is, because the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so-called Listen-Before-Talk, LBT, operation needs to be applied by the LTE node(s) before transmitting on a channel that uses unlicensed spectrum. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network, WLAN, standard known commercially as WI-FI.
IEEE 802.11 equipment uses a contention based medium access scheme. This scheme does not allow the wireless medium to be reserved at specific instances of time. Instead, IEEE 802.11 compliant devices only support the immediate reservation of the wireless medium following the transmission of at least one medium reservation message, e.g., a Request to Send, RTS, message or a Clear to Send, CTS, message. It is recognized herein that an analogous medium reservation signal is needed for LAA-LTE transmissions on both the downlink and the uplink, before the commencement of data transmission.
LTE uses Orthogonal Frequency Division Multiplexing or OFDM in the downlink and Discrete Fourier Transform, DFT, spread OFDM in the uplink, which is also referred to as Single-Carrier Frequency Division Multiple Access or SC-FDMA. The basic LTE downlink physical resource can be seen as a time-frequency grid, as illustrated in FIG. 1. The grid comprises Resource Elements or REs, where each RE corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink and the same number of SC-FDMA symbols in the time domain as OFDM symbols in the downlink.
In the time domain, LTE uplink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms. FIG. 2 illustrates this arrangement. For normal cyclic prefix or CP, one subframe consists of 14 SC-FDMA symbols. The duration of each symbol is approximately 71.4 μs.
Furthermore, the resource allocation in LTE is typically described in terms of Physical Resource Blocks, referred to as PRBs or simply RBs. One RB corresponds to one slot of 0.5 ms in the time domain and twelve contiguous subcarriers in the frequency domain. A pair of two adjacent RB in the time direction equals 1.0 ms and is known as a RB pair. RBs are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Uplink transmissions are dynamically scheduled, i.e., in each downlink subframe the base station transmits control information about which terminals should transmit data to the eNB in subsequent subframes, and which RBs to use for the data transmissions. Here, “eNB” denotes a LTE base station in the LTE Radio Access Network, RAN.
The uplink resource grid includes data and uplink control information for Physical Uplink Share Channel or PUSCH, transmissions, and includes uplink control information for Physical Uplink Control Channel or PUCCH transmissions, along with various reference signals. The reference signals include DeModulation Reference Signals or DMRS, and Sounding Reference signals or SRS. DMRS are used for coherent demodulation of PUSCH and PUCCH data, whereas SRS is not associated with any user traffic or control information but is generally used to estimate the uplink channel quality for purposes of frequency-selective scheduling.
An example uplink subframe is shown in FIG. 3. Note that UL DMRS and SRS are time-multiplexed into the UL subframe, and SRS are always transmitted in the last symbol of a normal UL subframe. The PUSCH DMRS is transmitted once every slot for subframes with normal cyclic prefix, and is located in the fourth and eleventh SC-FDMA symbols.
The subframes in which SRSs are transmitted by any UE within a cell are indicated by cell-specific broadcast signaling. A four-bit cell-specific “srsSubframeConfiguration” parameter indicates fifteen possible sets of subframes in which SRS may be transmitted within each radio frame. But, as noted, SRS transmissions are always in the last SC-FDMA symbol in the configured UL subframes, and PUSCH transmission may not be permitted on these symbols.
In the frequency domain, the SRS sequence for a particular UE is mapped to alternating subcarriers in a comb-like manner. This allows multiple UEs to simultaneously transmit SRS without overlap. The SRS sequence spans at least four RBs, and the maximum allowed bandwidth of one SRS is dependent on the UL system bandwidth and the cell-specific parameter srs-BandwidthConfig, CSRS∈{0, 1, . . . , 7}. For example, for an UL system bandwidth of 110 RBs and CSRS=0, the maximum possible SRS bandwidth for a particular UE is 96 RBs.
Further, different phase or cyclic shifts can be applied to SRS sequences on the same REs to make them mutually orthogonal. Up to eight such UE-specific shifts currently available per comb, according to the relevant LTE specifications. Thus, up to sixteen distinguishable full-BW SRS sequences can be assigned to UEs currently.
The LTE Release 10, Rel-10, standard supports bandwidths larger than 20 MHz. One important requirement on LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This requirement extends to spectrum compatibility. The need for spectrum compatibility means that an LTE Rel-10 carrier wider than 20 MHz should appear to a Rel-8 terminal as a number of LTE carriers. Each such carrier can be referred to as a Component Carrier or CC. For early LTE Rel-10 deployments it is expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals that predate the Rel-10 feature set. Therefore, it is necessary to assure that legacy terminals can efficiently use wide carriers. That is, a wideband carrier exceeding the Rel-8 carrier bandwidth should be structured so as to allow legacy terminals to be scheduled in all parts of the wideband carrier.
Carrier Aggregation or CA provides a straightforward mechanism to accomplish the desired compatibility. With CA, the aggregate or overall carrier bandwidth can exceed the capability of legacy terminals, but legacy terminals may be compatible with the individual CCs aggregated together to form the wideband carrier.
Correspondingly, this sort of CA configuration implies that a LTE Rel-10 terminal can receive multiple CCs, where each of CCs can have the same structure as a Rel-8 carrier. CA is illustrated in FIG. 4, which shows the aggregation of five 20 MHz CCs. A CA-capable UE is assigned a PCell, which is always activated, and may be further assigned one or more SCells. SCells may be activated or deactivated dynamically.
The number of aggregated CCs as well as the bandwidth of each CC may be different for the uplink and downlink. A symmetric configuration refers to the case where the number of CCs in the downlink and uplink are the same. Conversely, an asymmetric configuration refers to the case where the number of CCs in the downlink differs from the number of CCs in the uplink. Notably, the number of CCs configured in a given cell may be different from the number of CCs seen by a UE or other terminal. A terminal may, for example, support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
In typical WLAN deployments, Carrier Sense Multiple Access with Collision Avoidance, CSMA/CA, is used for medium access. A WLAN device uses CSMA/CA to sense whether the targeted channel is clear—i.e., to perform a Clear Channel Assessment or CCA. The WLAN device initiates a transmission on the WLAN channel only if only if the channel is deemed to be idle based on the CCA performed by the device. In case the channel is declared as busy, the transmission is essentially deferred until the channel is deemed to be idle. When the ranges of several WLAN Access Points, APs, using the same frequency overlap, all transmissions related to one AP might be deferred in case a transmission on the same frequency to or from another AP that is within range can be detected. Effectively, this circumstance means that if several APs are within range of each other, they will have to share a given channel in time, and the throughput for the individual APs may be severely degraded. FIG. 5 offers a generalized illustration of the LBT mechanism.
Several issues arise when a LTE network uses WLAN spectrum for LAA operation, because of the need for the LTE network to coexist with one or more other networks, systems or devices that also use all or part of the same unlicensed spectrum. Among other things, it is recognized herein that operating a LTE carrier in unlicensed spectrum according to the same conventions used for licensed-spectrum operation can seriously degrade the performance of any WI-FI system operating in the same unlicensed spectrum, as WI-FI access points and devices will not transmit on a channel once the channel is detected as being occupied.
One way for LTE to utilize unlicensed spectrum reliably is to transmit essential control signals and channels on a licensed carrier. FIG. 6 illustrates an approach to CA, where a UE is connected to a PCell in a licensed band and one or more SCells in an unlicensed band. The SCell operating in the unlicensed spectrum may be referred to as a “License Assisted Secondary Cell” or LA SCell.
For proper LAA operation, it is recognized herein that a node preparing to transmit on a channel having a channel frequency within the unlicensed spectrum, should first sense whether the channel is clear for transmission and should then immediately capture the channel so that other nodes or entities will not see the involved frequency or frequencies as being clear for use. However, it is recognized herein that current LTE specifications do not provide a mechanism for immediate channel capture. Indeed, the existing specifications constrain the ability of a LTE base station or terminal to perform immediate channel capture after a successful CCA on a channel occupying unlicensed spectrum.
In particular, existing LTE standards dictate when certain signals can be sent within the context of the overall frame, subframe, and slot timing of the radio signal, and do not define signaling that may be started at essentially arbitrary times. For example, in LTE, DMRS may be transmitted only in conjunction with scheduled PUSCH or PUCCH transmissions. SRS transmissions are also specified as being permitted only at certain times.